New membrane can block helium, yet allow water to flow freely

A separation membrane made of a stack of two dimensional layers of graphene …

Schematic of how graphene oxide membrane selectively allows water but nothing else through

Photograph by Science/AAAS

Membranes and barriers are used all the time in industrial and lab settings, and you may even have a few of them around the home. They can help keep materials apart that need to be separated, or can selectively allow certain materials to mix while holding others back. Graphene, the two-dimensional hexagonal lattice of carbon, is thought to be completely impermeable to all gases and liquids. That would obviously make it an extremely effective barrier film.

Creating sufficient quantities of pure graphene is not an an easy task, but graphene oxide (GO) films can be readily made in the lab. There's a paper in last week's edition of Science that examines the properties of graphene oxide films.

The authors examined the physical structure of the GO films using scanning electron microscopy and x-ray analysis. They confirmed that the films were layered structures consisting of individual crystallites a few microns in diameter (crystallites are the basic units of a polycrystalline material) with a five angstrom separation. The researchers created membranes of this material that ranged from 0.1 to 10 microns in thickness and measured about one centimeter in diameter. These membranes, even those with a thickness of less then a micron, were reasonably strong, withstanding differential pressures across them of up to 100 mbar.

Initial experiments with gases such as helium, hydrogen, nitrogen, and argon found that almost no gas was able to move across the membrane over a period of a few days. Calculations on these results yielded a helium permeation rate below 10-12 g/cm2*s*bar, consistent with values reported elsewhere for pure graphene films—basically no gas was getting through. Computing the bulk permeability of the material gave a value of approximately 10-15mm*g/cm2*s*bar. Put into more useful terms, this means that less gas will seep through a submicron thick GO film than would pass through a 1 mm thick glass barrier in an equivalent amount of time!

Carrying out a similar experiment with common liquids (ethanol, hexane, acetone, decane, and propanol) revealed that no weight loss could be detected after several days of the fluid resting on the membrane. This set an upper limit on liquid bulk permeability of 10-11 mm*g/cm2*s*bar.

However, something unexpected happened when they repeated the test with water. There was a huge weight and the evaporation rate was nearly the same as though there was no membrane or barrier in place.

After this unexpected result, the authors repeated the test with helium to ensure that no physical damage had occurred to the membrane; no helium leakage was observed. In fact, it was only when the membrane was a couple of microns thick that any resistance to the flow of water was observed. Computing the bulk permeability of water gave a result of 10-5 mm*g/cm2*s*bar, a value 1010 (10,000,000,000) times greater than that for helium. This membrane was essentially impermeable to a small, inert gas, but allowed water to freely move through it.

To get a better idea of what was happening, the team carried out permeation experiments using mixtures of water with other gases and liquids. In the presence of saturated water vapor, helium was able to traverse the membrane, albeit five orders of magnitude more slowly than the water. The authors noted that the rate at which helium moved through was actually the same rate at which helium would diffuse through a similar column of water, suggesting that it was diffusing through the freely moving water.

Another series of experiments compared the rate of water permeation through the membrane to the rate at which water evaporated in an open-ended system. By varying the pressure across the membrane and the relative humidity in the test chamber, the researchers determined that water was limited only by the rate of mass transfer at the far surface—the rate of evaporation after the water had moved across the membrane was limiting how quickly it could move through the membrane. There was a molecular traffic jam on the far end holding up the entire process.

To explain how this was possible, the authors hypothesized that the size between the layers of graphene were just right. They suggest that, at a free channel spacing of just about five angstroms, a monolayer of water forms that is capable of undergoing a low-friction flow in the two-dimension channels that exist between the layers (take that, no slip hypothesis!). To move between layers—and hence traverse the membrane—the authors posit that a percolating network of graphene nanocapillaries exists that allows the water to flow throughout and across the membrane.

To test this hypothesis, the authors carried out a handful of experiments. First, they reduced the graphene oxide material by calcining it in a 250oC hydrogen-argon atmosphere. This resulted in a reduction of water permeability by a factor of 100. The authors attribute this to the reduction in the channel width; X-ray examination of the material showed it was reduced to about four angstroms, which is too small for a monolayer of water to form and flow.

To further back up the idea that the high water permeability was due to the just-right size of the channel, the authors ran a series of molecular dynamics simulations that modeled how a reservoir of water molecules would move through a single, two-dimensional graphene pore. By varying the pore width, they showed that water could not form a monolayer and flow in pores smaller than six angstroms. Above 10 angstroms, a second monolayer would form that would greatly reduce the flow. It was the intermediate region that allowed for a low-friction flow of a monolayer of water.

While this represents an interesting result with a wide variety of applications—water removal, water filtration, degassing, and dewetting in industrial settings—an accompanying perspective in the same issue of Science puts the discovery, well, in perspective. Any new industrial use of such discoveries is a long way off, because we first need to learn how to make larger membranes (the ones used in the article are ~1 cm across). They'll also need to stand up to industrial scale pressures, and be packaged into a useful device. While these are important discoveries, their immediate uses will be limited to small scale and niche applications.

This is amazing. Now THIS is science at its best, and with potential application. What that application might be I am too small-minded to say, but taking this idea, figuring out exactly how it works... we might be able to figure out how to manufacture other, more targeted "filters" (for lack of a better term). This is neat!

So I ferment a batch of whatever my favorite concoction of the day is, filter out the particulates, place remainder in a jar with a sheet of graphene oxide (GO) film over the top, and I've got a still? Granted it might be slower than the standard Ozark Special but the results sound like they'd be the same.

Earlier in the article you say the substance they tested had a 5 angstrom gap and worked by allowing a water monolayer to form. Then later in the article you say they tested and found that monolayers only form between 6 and 10 angstroms. Probably not a big deal, just makes me feel a little doubtful about the technical accuracy of the article.

A bunch of the references were to the liquid evaporating out through the membrane, as opposed to while still in the liquid state. Does the water have to be a vapor for this to work? (I'm assuming yes, please correct me if I missed something)

I would imagine this type of thing being useful down the road in fuel cells. Very cool article.

place remainder in a jar with a sheet of graphene oxide (GO) films over the top, and I've got a still?

Yes! In fact, the researchers did that. "Just for a laugh, we sealed a bottle of vodka with our membranes and found that the distilled solution became stronger and stronger with time." A shame Ars left out this fabulous detail.

Also, add something to condense the outgoing water vapor and graphene sheets could revolutionize water purification.

This is amazing in two directions! I work with high vacuum systems and hydrogen is always getting in, through thick steel & aluminum walls! The idea that a thin layer of anything would be so impermeable is awesome. Then to think that water just slips through? Amazing! The physics they investigated make perfect sense but the result is seemingly as miraculous as the Lotus effect.

Does this in any way have practical applications with building a fleet of Zeppelins? A friend wanted to know.

Hi Folks, This discovery could reduce the cost of higher purity Helium, as over half the cost of producing high purity Helium that is required by Zeppelins is due to the cost of purification from the initial balloon only grade Helium that is obtained after initial liquifraction of natural gas. No one serious is going to pay for new Zeppelins as they cost nearly twice as much as a normal blimp type airships. Modern airships have very low leak rates so Helium is a very minor part of running costs. Regards Joe (Gasbags comedy: www.hybridblimp.net )

place remainder in a jar with a sheet of graphene oxide (GO) films over the top, and I've got a still?

Yes! In fact, the researchers did that. "Just for a laugh, we sealed a bottle of vodka with our membranes and found that the distilled solution became stronger and stronger with time." A shame Ars left out this fabulous detail.

Also, add something to condense the outgoing water vapor and graphene sheets could revolutionize water purification.

Now i am envisioning a large sewage tank or something exposed to sunlight but capped with such a membrane. Water evaporates off, pass thru membrane and is drinkable on the other side. Hell, maybe it can be used to recycle water in space?

This is amazing in two directions! I work with high vacuum systems and hydrogen is always getting in, through thick steel & aluminum walls! The idea that a thin layer of anything would be so impermeable is awesome. Then to think that water just slips through? Amazing! The physics they investigated make perfect sense but the result is seemingly as miraculous as the Lotus effect.

And the other way round, this could perhaps make long term storage of hydrogen in pure form more interesting. Right now it seems that one either need to cool it to liquid form (and even then there will be some evaporation loss) or bind it in some kind of salt.

place remainder in a jar with a sheet of graphene oxide (GO) films over the top, and I've got a still?

Yes! In fact, the researchers did that. "Just for a laugh, we sealed a bottle of vodka with our membranes and found that the distilled solution became stronger and stronger with time." A shame Ars left out this fabulous detail.

Also, add something to condense the outgoing water vapor and graphene sheets could revolutionize water purification.

Now i am envisioning a large sewage tank or something exposed to sunlight but capped with such a membrane. Water evaporates off, pass thru membrane and is drinkable on the other side. Hell, maybe it can be used to recycle water in space?

That would be a brilliant usage of it, as supposedly if all it let through were H20 molecules it would be just water.

Does this in any way have practical applications with building a fleet of Zeppelins? A friend wanted to know.

Hi Folks, This discovery could reduce the cost of higher purity Helium, as over half the cost of producing high purity Helium that is required by Zeppelins is due to the cost of purification from the initial balloon only grade Helium that is obtained after initial liquifraction of natural gas. No one serious is going to pay for new Zeppelins as they cost nearly twice as much as a normal blimp type airships. Modern airships have very low leak rates so Helium is a very minor part of running costs. Regards Joe (Gasbags comedy: http://www.hybridblimp.net )

Zepplins have historically always used hydrogen instead of helium. Hydrogen is still relatively easy to and cheap to produce.

This would be fun with separating the fuel from water in the algea fuel farm schemes. And do not forget you can put a different filter in front of a this GO filter so you can increase the lifetime of the more delicate filter. I also think we should just make a honeycomb patterned filter of 1cm NO cells for now and leave making them bigger for the smarter future versions of ourselves.

So I ferment a batch of whatever my favorite concoction of the day is, filter out the particulates, place remainder in a jar with a sheet of graphene oxide (GO) film over the top, and I've got a still? Granted it might be slower than the standard Ozark Special but the results sound like they'd be the same.

Edit: I suck at proofreading I suppose. Fixed.

You'd have a type of still, but the difference would be noticeable for some types of liquors. Stills don't just reduce the water content, they also can modify the ratios of other flavor components in the final product.

So, could this process conceivably be developed to regulate biological functions, e.g. enzyme manufacture/transport, or is the H2O monolayer aspect the primary thing we should be focusing on with this nascent technology?

place remainder in a jar with a sheet of graphene oxide (GO) films over the top, and I've got a still?

Yes! In fact, the researchers did that. "Just for a laugh, we sealed a bottle of vodka with our membranes and found that the distilled solution became stronger and stronger with time." A shame Ars left out this fabulous detail.

Also, add something to condense the outgoing water vapor and graphene sheets could revolutionize water purification.

Now i am envisioning a large sewage tank or something exposed to sunlight but capped with such a membrane. Water evaporates off, pass thru membrane and is drinkable on the other side. Hell, maybe it can be used to recycle water in space?

I don't know how practical it would be to build a wastewater recycling system that depends on evaporation for the water reclamation -- evaporation is normally slow, and takes a lot of energy to speed up -- but it's definitely an interesting idea to play around with. In areas where water is relatively inaccessible you could potentially build a closed system for indefinite water reuse, so that you'd only have to haul in enough water once (other than covering for whatever modest loss rate your not-totally-closed system has).

Earlier in the article you say the substance they tested had a 5 angstrom gap and worked by allowing a water monolayer to form. Then later in the article you say they tested and found that monolayers only form between 6 and 10 angstroms. Probably not a big deal, just makes me feel a little doubtful about the technical accuracy of the article.

Sorry for not being clearer. There are two measurements that are getting thrown around. The center-to-center distance between the carbons atoms in the adjacent graphene layers (6 to 10 angstroms) and the actual (effective) floor-to-ceiling distance that molecules would have to move through the pores (5 or so angstroms)

Matt Ford / Matt is a contributing writer at Ars Technica, focusing on physics, astronomy, chemistry, mathematics, and engineering. When he's not writing, he works on realtime models of large-scale engineering systems.